If someone describes you as a pillar of strength, he's saying you're reliable and supportive, much like a pillar or column of a building that helps hold the structure up.
The spelling of the word pillar looks like it has two pillars right at its center, holding the word up. Pillar is interchangeable with the word column, though you can't always use them in the same contexts. While a column and a pillar are both physical cylindrical structures, a column can also be the row running up and down on a chart, while a pillar is often someone or thing that is considered a foundation or support. Someone indispensable to your company might be considered a pillar to the organization.